In order to, perform electronic control of the variable valve timing control, the variable valve timing controller which has the motor as a source of the drive has been developed. The variable valve timing controller described in JP-2006-70754A (US2006/0042578A1) includes a first gear, a second gear, a phase changing gear, and an electric motor. The first gear (outer gear) is concentrically arranged with the camshaft and is rotated with the rotation driving force of the crankshaft. The second gear (inner gear) rotates together with the camshaft. The phase changing gear (planet gear) transmits the torque of the first gear to the second gear, and varies the rotational phase of the second gear relative to the first gear. The motor is coaxially provided to the camshaft so that the revolution speed of the phase changing gear is controlled. The number of teeth of the first gear, the second gear, and the phase changing gear is determined so that the camshaft may rotate with one half of the rotational speed of the rotational speed of the crankshaft. And when not varying valve timing, the rotational speed of the motor is coincided with the rotational speed of the camshaft, and the revolution speed of the phase changing gear is coincided with the rotational speed of the camshaft. When varying valve timing, the motor speed is varied relative to the rotational speed of the camshaft, and the revolution speed of the phase changing gear is varied relative to the rotational speed of the camshaft. Thereby the difference between the rotational phase of the first gear and the second gear is varied.
When controlling the motor, a control signal indicative of a target motor speed is outputted to a motor driving circuit from an engine control unit. A duty ratio of switching element, such as a power MOSFET, is adjusted to control an output current to the motor from the motor driving circuit. The motor driving circuit receives supply voltage from a battery. There are some cases in which the supply voltage is decreased due to a shortage of charging or an increase of power consumption of in-vehicle equipments. In a case that the supply voltage is decreased to a minimum operation assurance voltage for a long period, there is a possibility that the switching elements generate heat to cause a malfunction thereof.